1. Technical Field
This invention relates in general to telecommunications and, more particularly, to protection rings.
2. Description of the Related Art
Over the last decade, the importance of telecommunications has increased dramatically. In order to accommodate the vast amount of information passed over telecommunications systems, such as the Public Switched Telephone Network (PSTN) and data networks, copper wires are being replaced with optical fibers, which are capable of carrying significantly more information.
A single fiber may transmit information over many different channels using DWDM (dense wavelength division multiplexing) techniques. Improvements in fiber technology and electronics are increasing the number of channels that may be distinguished over a fiber and, thus, the amount of information that may be passed by a single fiber.
Increases in information bandwidth over a fiber, however, increase the importance of providing mechanisms to bypass failures in the network, until the failure can be corrected. Common failures include, for example, fiber breakages (typically caused by construction activities inadvertently cutting a fiber), fiber disconnects caused by accidents in the central office, and network element failures, such as laser failures.
In order to maintain communications in spite of a failure, ring architectures are often used. In a ring architecture, a series of network elements are connected in a ring, such as shown in FIG. 1a. Each ring 10 has multiple network elements 12 coupled to one another to form a closed loop. Typically, there are four fibers 14 connecting adjacent network elements 10—two working fibers and two protection fibers, although other configurations are possible. The working fibers (W) carry traffic between adjacent nodes. Protection fibers (P) are available to carry traffic in the event of a working fiber failure. The protection fibers also convey control information between network elements; when not being used for traffic, the protection fibers may carry low-priority interruptible traffic.
The ring architecture shown in FIG. 1a is a very simple architecture. In many circumstances, multiple rings 10 may connect various network elements 12 as shown in FIG. 1b. As shown in FIG. 1a, network elements 12 may be shared between different rings. Failures of a working fiber in any of the rings 10 may cause protect lines in multiple rings to be used.
FIG. 2a illustrates one prior art method of circumventing a failure of a working fiber W. In this embodiment, a ring 10 having five network elements 12 (referenced individually as network elements 12a-12e) has a broken working fiber W between network elements 12c and 12d. For purposes of illustration, only one working fiber W and one protection fiber P is shown, it being understood that a similar pair of working and protection fibers are used for traffic in the opposite direction. To pass traffic between network elements 12c and 12d, network element 12d connects the working lines 16de to protect lines 18cd and network element 12c connects working lines 16bc to protect lines 18cd. In other words, traffic that would normally be routed over working lines 16cd is switched to the associated protect lines 18cd. This is referred to as a “span” switch.
FIG. 2b illustrates a situation where both the working and protection lines have failed between network elements 12c and 12d. In this case, a “ring” switch is implemented where working line 16de is rerouted to protect line 18de and working line 16bc is rerouted to protect line 18bc. Accordingly, the remaining viable protect lines all carry traffic. Every network element can still communicate with all the other network elements 12 on the ring.
In traditional SONET/SDH applications, the transport mechanism protocol uses K1/K2 overhead bytes transmitted “in-band”, i.e., on the same channel as the associated traffic. There is an increasing demand to use “transparent” network elements, which do not translate the data carried on the optical fibers to electronic form; instead the data traffic is switched at the network element using an optical switch. While transparent network elements have the benefit of increased speed through the switch, it is not possible to add control data to the channels. Hence, it is not possible to use the current standard K1/K2 overhead bytes.
In addition, shared protection structures provide additional protection capabilities. A basic shared ring architecture is disclosed in WO 99/23773 (PCT/IB98/01955) to Elahmadi et al, where two rings share a protection path between a pair of common network elements. This application proposes the use of a single physical span (the shared protect line) between the common network elements to provide protection for the two rings. A failure on either ring can be remedied by using the shared protect line to carry traffic. This architecture reduces costs, which can be significant if the distance between the shared network elements is long (or there are other infrastructure costs involved), but increases the chance of a traffic outage on one ring if a failure occurs while there is another failure on another ring.
U.S. Ser. No. 09/903,268, entitled “Method and Apparatus for Signaling in a Shared Protection Ring Architecture”, to Mazzurco et al, U.S. Ser. No. 09/858,099, entitled “Common Protection Architecture For Optical Network”, to Mazzurco et al, and U.S. Ser. No. 09/858,098, entitled “Optical Shared Protection Ring For Multiple Spans”, to Mazzurco et al, all of which are incorporated by reference herein, propose additional features for shared ring structures. These features are not supported by traditional K-byte protection schemes.
Recognizing these problems, the IETF (Internet Engineering Task Force) has generated a proposal for an Optical Automatic Protection Switching protocol (O-APS) for optical rings. This proposal uses the OSC (optical supervision channel) of each link for passing protection signaling. The O-APS protocol in the IETF draft combines the functions of a signaling engine with that of a ring state machine processor. The protocol calls out specific protection switching procedures that are different from traditional, time-tested, switching procedures. The switching procedures are tightly coupled to the passing of the protection signaling data in a single protocol. While the proposed protocol uses information similar to K-bytes for the SONET/SDH standards, the data is encoded in a completely different format.
Making vast changes in the switching procedures and associated data is almost certain to lead to errors, and can add significantly to the cost of developing the equipment.
Therefore, a need has arisen for a protection signaling architecture that takes advantage of previously developed protocols and enables enhanced features for shared ring architectures.